Electrical circuits (e.g., circuit boards) may be tested both during and after manufacture. Circuits can be tested to identify both component defects and assembly defects.
Some components, such as capacitors and batteries, are capable of storing energy. When automated test equipment (ATE) is coupled to one of these components (either directly or indirectly), the component can discharge its energy and potentially damage the ATE or other components of the circuit under test. Components such as capacitors can be especially dangerous, because their states are more likely to change between test processes (whereas batteries are more likely to remain charged throughout testing).
One approach for protecting ATE from energy discharge is to provide ATE with cross-connect relays with higher current ratings. However, this approach does not protect circuits under test from short circuit damage. Nor does it reduce wear on ATE resources. The higher rated components also add additional cost.
Another approach, for protecting both ATE and circuits under test from energy discharge, is to incorporate algorithms for identifying potential energy sources into test development software (e.g., by examining board topology). Software can then be used to write tests that cause the potential energy sources to discharge prior to testing. One weakness of this approach, however, is that it often depends on human operators to provide correct inputs, such as correct circuit topology information.
Yet another approach for protecting ATE and circuits under test is to incorporate one or more discharge mechanisms into a custom fixture for each circuit under test.
A typical problem with all of these approaches is that stray charges may “walk” around the board. That is, after discharging one electrical net, a non-discharged energy source may cause energy to leak back into the already discharged electrical net. And, even if a stray charge is small enough that it does not damage ATE or other circuitry, it may be significant enough to cause errors in circuit test measurements—especially when a circuit is submitted to low voltage and low current tests.
Also, the last two of the above approaches are susceptible to human error. That is, their approaches vary based on circuit topology, and therefore require different methodologies for different circuits under test.